Evidence that Neptune has one or more rings arose in the mid-1980s when stellar occultation studies from Earth occasionally showed a brief dip in the star’s brightness just before or after the planet passed in front of it. Because dips were seen only in some studies and never symmetrically on both sides of the planet, scientists concluded that any rings present do not completely encircle Neptune but instead have the form of partial rings, or ring arcs.

Images from Voyager 2, however, revealed a system of six rings, each of which in fact fully surrounds Neptune. The putative arcs turned out to be bright regions in the outermost ring, named Adams, where the density of ring particles is particularly high. Although rings also encircle each of the other three giant planets, none displays the striking clumpiness of Adams. The arcs are found within a 45° segment of the ring. From leading to trailing, the most prominent are named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité. They range in length from about 1,000 km (600 miles) to more than 10,000 km (6,000 miles). Although the moon Galatea, which orbits just planetward of the inner edge of Adams, may gravitationally interact with the ring to trap ring particles temporarily in such arclike regions, collisions between ring particles should eventually spread the constituent material relatively uniformly around the ring. Consequently, it is suspected that the event that supplied the material for Adams’s enigmatic arcs—perhaps the breakup of a small moon—occurred within the past few thousand years.

The other five known rings of Neptune—Galle, Le Verrier, Lassell, Arago, and Galatea, in order of increasing distance from the planet—lack the nonuniformity in density exhibited by Adams. Le Verrier, which is about 110 km (70 miles) in radial width, closely resembles the nonarc regions of Adams. Similar to the relationship between the moon Galatea and the ring Adams, the moon Despina orbits Neptune just planetward of the ring Le Verrier. Each moon may gravitationally repel particles near the inner edge of its respective ring, acting as a shepherd moon to keep ring material from spreading inward. (For fuller treatments of shepherding effects, seeSaturn: Moons: Orbital and rotational dynamics; Uranus: The ring system.)

Galle, the innermost ring, is much broader and fainter than either Adams or Le Verrier, possibly owing to the absence of a nearby moon that could provide a strong shepherding effect. Lassell consists of a faint plateau of ring material that extends outward from Le Verrier about halfway to Adams. Arago is the name used to distinguish a narrow, relatively bright region at the outer edge of Lassell. Galatea is the name generally used to refer to a faint ring of material spread all along the orbit of the moon Galatea.

Rings of Neptune

name

distance from centre of planet (km)

observed width (km)

comments

Galle

41,900

2,000

indistinct edges

Le Verrier

53,200

110

flanked at inner edge by moon Despina

Lassell

55,200

4,000

bounded by rings Le Verrier and Arago

Arago

57,200

less than 100

somewhat brighter outer edge of broad Lassell ring

Galatea

61,950

less than 100

co-orbital with moon Galatea

Adams

62,930

15

possesses bright arcs; flanked at inner edge by moon Galatea

None of Neptune’s rings were detected from scattering effects on Voyager’s radio signal propagating through the rings, which indicates that they are nearly devoid of particles in the centimetre size range or larger. The fact that the rings were most visible in Voyager images when backlit by sunlight implies that they are largely populated by dust-sized particles, which scatter light forward much better than back toward the Sun and Earth. Their chemical makeup is not known, but, like the rings of Uranus, the surfaces of Neptune’s ring particles (and possibly the particles in their entirety) may be composed of radiation-darkened methane ices.

The suspected youthfulness of Adams’s ring arcs and the arguments offered can be extended to Neptune’s rings in general. The present rings are narrow, and scientists have found it difficult to explain how the orbits of the known moons can effectively confine the natural radial spreading of the rings. This has led many to speculate that Neptune’s present rings may be much younger than the planet itself, perhaps substantially less than a million years. The present ring system may be markedly different from any that existed a million years ago. It is even possible that the next spacecraft to visit Neptune’s rings will find a system greatly evolved from the one Voyager 2 imaged in 1989.

Observations from Earth

Neptune’s discovery

Neptune is the only giant planet that is not visible without a telescope. Having an apparent magnitude of 7.8, it is approximately one-fifth as bright as the faintest stars visible to the unaided eye. Hence, it is fairly certain that there were no observations of Neptune prior to the use of telescopes. Galileo is credited as the first person to view the heavens with a telescope in 1609. His sketches from a few years later, the first of which was made on Dec. 28, 1612, suggest that he saw Neptune when it passed near Jupiter but did not recognize it as a planet.

Prior to the discovery of Uranus by the English astronomer William Herschel in 1781, the consensus among scientists and philosophers alike was that the planets in the solar system were limited to six—Earth plus those five planets that had been observed in the sky since ancient times. Knowledge of a seventh planet almost immediately led astronomers and others to suspect the existence of still more planetary bodies. Additional impetus came from a mathematical curiosity that has come to be known as Bode’s law, or the Titius-Bode law. In 1766 Johann Daniel Titius of Germany noted that the then-known planets formed an orderly progression in mean distance from the Sun that could be expressed as a simple mathematical equation. In astronomical units (AU; the mean Sun-Earth distance), Mercury’s distance is very nearly 0.4; the distances of Venus, Earth, Mars, Jupiter, and Saturn are approximately 0.4 + (0.3 × 2n), in which n is 0, 1, 2, 4, and 5, respectively, for the five planets. The astronomer Johann Elert Bode, also of Germany, published the law in 1772 in a popular introductory astronomy book, proposing that the missing 3 in the progression might indicate an as-yet-undiscovered planet between Mars and Jupiter.

The suggestion was met with little enthusiasm until the mean distance of Uranus, at 19.2 AU, was noted to be very nearly equal to that predicted by Bode’s law (19.6 AU) for n = 6. Moreover, when the first asteroids, beginning with the discovery of Ceres in 1801, were found to be in orbit between Mars and Jupiter, they satisfied the n = 3 case of the equation.

Some astronomers were so impressed by the seeming success of Bode’s law that they proposed the name Ophion for the large planet that the law told them must lie beyond Uranus for the n = 7 case, at a distance of 38.8 AU. In addition to this scientifically unfounded prediction, observations of Uranus provided actual evidence for the existence of another planet. Uranus was not following the path predicted by Newton’s laws of motion and the gravitational forces exerted by the Sun and the known planets. Furthermore, more than 20 recorded prediscovery sightings of Uranus dating back as far as 1690 disagreed with the calculated positions of Uranus for the respective time at which each observation was made. It appeared possible that the gravitational attraction of an undiscovered planet was perturbing the orbit of Uranus.

In 1843 the British mathematician John Couch Adams began a serious study to see if he could predict the location of a more distant planet that would account for the strange motions of Uranus. Adams communicated his results to the astronomer royal, George B. Airy, at Greenwich Observatory, but they apparently were considered not precise enough to begin a reasonably concise search for the new planet. In 1845 Urbain-Jean-Joseph Le Verrier of France, unaware of Adams’s efforts in Britain, began a similar study of his own.

By mid-1846 the English astronomer John Herschel, son of William Herschel, had expressed his opinion that the mathematical studies under way could well lead to the discovery of a new planet. Airy, convinced by Herschel’s arguments, proposed a search based on Adams’s calculations to James Challis at Cambridge Observatory. Challis began a systematic examination of a large area of sky surrounding Adams’s predicted location. The search was slow and tedious because Challis had no detailed maps of the dim stars in the area where the new planet was predicted. He would draw charts of the stars he observed and then compare them with the same region several nights later to see if any had moved.

Le Verrier also had difficulty convincing astronomers in his country that a telescopic search of the skies in the area he predicted for the new planet was not a waste of time. On September 23, 1846, he communicated his results to the German astronomer Johann Gottfried Galle at the Berlin Observatory. Galle and his assistant Heinrich Louis d’Arrest had access to detailed star maps of the sky painstakingly constructed to aid in the search for new asteroids. Galle and d’Arrest identified Neptune as an uncharted star that same night and verified the next night that it had moved relative to the background stars.

Although Galle and d’Arrest have the distinction of having been the first individuals to identify Neptune in the night sky, credit for its “discovery” arguably belongs to Le Verrier for his calculations of Neptune’s direction in the sky. At first the French attempted to proclaim Le Verrier as the sole discoverer of the new planet and even suggested that the planet be named after him. The proposal was not favourably received outside France, both because of Adams’s reported contribution and because of the general reluctance to name a major planet after a living individual. Neptune’s discovery was eventually credited to both Adams and Le Verrier, although it now appears likely that Adams’s contribution was less substantial than earlier believed. It is nevertheless appropriate that the more traditional practice of using names from ancient mythology for planets eventually prevailed.

The discovery of Neptune finally laid Bode’s law to rest. Instead of being near the predicted 38.8 AU, Neptune was found to be only 30.1 AU from the Sun. This discrepancy, combined with the lack of any scientific explanation as to why the law should work, discredited it. The discovery in 1930 of Pluto, regarded as the ninth planet at the time, at a distance of 39.5 AU was even more at variance with the equation’s prediction of 77.2 AU for n = 8. Not even the proximity of Pluto’s mean distance to the 38.8 AU predicted for n = 7 could resurrect the credibility of Bode’s law.

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